Option to keep downloaded tarballs

[meejah]

I just had two hash failures in a peep-install and would have loved to see what's different in the tarballs, but I don't see any promising option to not delete the tempdir; does one exist already?

If not, hopefully I can come up with a patch.

[erikrose]

Nope! I'd take a patch.

[meejah]

See #46